go RE. Clements 



the west (Clements, 1920; Weaver and Clements, 1929). The last 

 two are to be considered as climatic differentiations of the mixed 

 prairie, arising from the same causes that led to the development 

 of desert in an originally more or less uniform vegetation. This 

 is indicated by the fact that the desert plains of southern Arizona 

 and northern Mexico today swing north as a border along the 

 Colorado Desert. Hence, the origin of the desert vegetation is 

 inseparably bound up with the disappearance of the grass domi- 

 nants in the region of desiccation and their realignment in the 

 areas round about. 



The typical dominant of the desert climax, Larrea tridentata, 

 resembles the sagebrush, Artemisia tridentata, in its distribution 

 over an area much wider than that in which it is climax. In the 

 United States it is found from western Texas to the eastern edge 

 of the Great Valley of California but, of all this vast stretch, it is 

 climax only in western Arizona and southeastern California, 

 as well as in Mexico. In the wake of overgrazing, it has spread 

 extensively to replace the vanishing grasses, and today its proper 

 climatic region can only be determined by the presence of reUct 

 grasses on the one hand and of its peculiar desert associates 

 on the other. As in various other climaxes, disturbance pro- 

 cesses have produced a misleading picture, the true significance 

 of which can be discovered only by turning to the dynamics 

 involved. This demands not merely a proper understanding of 

 the effects of disturbance by man, catde, and rodents, but like- 

 wise an insight into the direction and influence of minor and 

 major climatic shifts. Both bio-ecology and paleo-ecology must 

 be called upon to provide a complete synthesis and to permit a 

 reconstruction of past biomes, as well as to trace the develop- 

 mental processes active in producing the present vegetation. 

 These are the objects of the present paper. 



